1. Field of the Invention
The present invention relates to electrochemical gas sensors and, more specifically, to electrochemical gas sensors capable of sensing low partial pressure of a target gas. The present invention also relates to methods for sensing gas partial pressure, particularly at a low level of target gas.
2. Description of the Invention Background
Electrochemical gas sensors are used to sense a target gas content of a sample gas. Although the description of known electrochemical gas sensors in this application is directed towards the improvement of oxygen sensors in particular, the discussion is applicable to any type of electrochemical gas sensor.
A desirable electrochemical gas sensor should:
1. Provide zero output signal (zero offset) when target gas partial pressure (or concentration) is zero. This feature is more important when sensing a low level of a target gas, for example, in the 0-100 parts per billion range. PA1 2. React rapidly to changes in target gas partial pressure. PA1 3. Provide stable and repeatable output signals at all target gas partial pressures.
Typical electrochemical sensors include an anode and a cathode disposed in an electrolyte contained within a sensor body (cell). In a typical oxygen sensor, the output signal increases with the oxygen partial pressure in the sample gas. However, at near zero oxygen partial pressure levels, the output signal of the typical oxygen sensor often deviates significantly from the correct value due to a zero offset. The major contributing factors to this zero offset are impurities, including byproducts of the electrochemical reactions, that accumulate on the electrodes. Ionic impurities in the electrolyte or impurities that originate from the electrodes can lead to secondary electrochemical reactions at the electrodes. Upon deposition on the electrode, those impurities may also catalyze the reduction of hydrogen ions on the cathode. Those reactions are generally independent of the oxygen reduction process at the cathode generating sensor outputs even in the absence of oxygen.
Typical high sensitivity (ppm or ppb) electrochemical oxygen sensors are slow to recover their sensitivity after exposure to gases with a high oxygen partial pressure. During exposure to high oxygen partial pressure, the level of dissolved oxygen in the sensor electrolyte increases as a result of diffusion of oxygen from the sample gas into the electrolyte. Dissolved oxygen is reduced at the cathode in exactly the same manner as the oxygen from the sample gas thereby distorting the value of electrical current produced by the oxygen from the sample gas. The contribution to sensor current by dissolved oxygen generally decays with time. For typical electrochemical oxygen sensors, the time required for the sensor output to recover back to below a 1 ppm level after sensor exposure to atmospheric air could range from hours to days. To minimize the contributions of dissolved oxygen to the sensor output signal, oxygen free gases (zero gases) are often injected into and allowed to bubble through the electrolyte to reduce the level of dissolved oxygen in the electrolyte.
Since the output from an electrochemical oxygen sensor is dependent on the excitation potential at the cathode, a steady excitation potential is essential to achieving a stable and repeatable sensor output. For electrochemical oxygen sensors that are based on the fuel cell reactions, the excitation potential is derived from the redox reactions involving the anode material. There are many contributing factors to varying excitation potentials at the cathode of an oxygen sensor. For example, variations in temperature, redox reactant concentrations, and electrolyte electrical conductivity could affect the cathode potential. To overcome this issue, external power sources are often used to generate the necessary cathode potential. Circuitry such as a potentiostat circuit have been adopted by many skilled in the trade to provide the appropriate potentials to electrochemical sensors. The potentiostat approach necessitates the usage of high impedance op-amps and three electrodes for the sensor: the working electrode, the reference electrode, and the auxiliary (counter) electrode. For an oxygen sensor, oxygen is reduced at the working electrode. The reference electrode, carrying essentially no current, ensures that the excitation potential of the cathode remains constant. The potential of the auxiliary electrode will be automatically adjusted, through the potentiostat circuitry, to complete the redox reactions and maintain the working electrode potential. Although the potentiostat approach improves over a two electrode design by providing a more stable cathode excitation potential, it also does not address the zero offset and dissolved oxygen issues.
The foregoing problems associated with prior electrochemical gas sensors contribute to inaccurate readings as well as time varying sensor outputs. These problems are most pronounced when sensing low partial pressure of a target gas in the parts per billion (ppb) range. Accordingly, a need exists for an improved electrochemical gas sensor and for a target gas sensing method that permits accurate measurement of target gas partial pressure.
A galvanic or polarographic, electrochemical oxygen sensor includes an anode and a cathode disposed in an electrolyte within a sensor body. The electrical current flowing between the anode and cathode varies with the partial pressure of target gas present in the sample gas. The sensor detects oxygen as a result of electrochemical reduction of oxygen at the cathode (sensing electrode). For this to occur, an excitation electrical potential must be applied to the sensing electrode. This potential is generally selected so that all oxygen molecules reaching the sensing electrode are reduced immediately.
In a polarographic sensor, an anode and an external bias power supply are used to provide potential. A typical polarographic oxygen sensor employs a Ag/AgCl or Ag/Ag.sub.2 O electrode (anode) along with an external bias potential of -0.7 to -0.8 V to drive the oxygen reduction at the sensing electrode. During the sensing process, oxygen is reduced at the sensing electrode while a proportional amount of silver is oxidized at the anode. AgCl and Ag.sub.2 O are insoluble in the electrolyte and precipitate as an insulating thin film on the surface of the anode, limiting the Ag electrode from further participation in the electrochemical reactions. As a result, a large silver electrode is required in order to obtain a stable potential and a reasonable life span. The zero offset of such a polarographic sensor, due to the presence of dissolved oxygen and trace amount of Ag.sup.+ ions in the electrolyte, is generally too high for measurement of oxygen partial pressure in the parts per million range.
A known galvanic oxygen sensor includes a lead electrode with zero external potential used to drive the oxygen reduction at the sensing electrode. The lead anode alone provides a suitable potential for oxygen reduction. However, sensors constructed this way suffer from long term output drift and excessive noise at the parts per billion oxygen level as a result of gradual accumulation of lead ions in the electrolyte. Cadmium, having a much lower solubility in the electrolyte, is often used to replace lead as an electrode material. Unfortunately, the excitation potential provided by the cadmium is not ideal for the oxygen reduction at the sensing electrode.
An oxygen sensor using two gas diffusion electrodes is also know in the art. A bias potential applied to these electrodes causes oxygen reduction at the sensing electrode and oxidation of water from the electrolyte at the anode. The advantage of this approach is that no by-product is generated during the oxygen sensing process. As a result, the composition of the electrolyte remains unchanged. The drawback of this approach is that the oxidation of water at the anode is sensitive to environmental conditions. Consequently, the anode in this arrangement is unable to provide a stable potential for the reduction of oxygen at the sensing electrode. This in turn limits this approach's oxygen reading consistency and accuracy at ppb levels.
Another known sensor replaces the water/oxygen anode with a hydrogen anode to prevent oxidation of water at the anode. However, the need to use hydrogen during operation is a nuisance and can be a safety hazard.
A sensor having an Ni(OH).sub.2 /NiOOH anode is also known and used because such anode materials have low solubility in electrolyte. The low solubility of the Ni(OH).sub.2 and NiOOH, however, causes current through the anode to be blocked in a way similar to the blockage problems encountered with the Ag/AgCl, Ag/Ag.sub.2 O, and cadmium electrodes discussed earlier.
A known trace oxygen sensor that uses zero gas to purge the electrolyte in order to remove the dissolved oxygen creates a significant noise problem in the sensor output and often requires an expensive oxygen scrubber to produce the zero gas.
A method of removing dissolved oxygen in sensor electrolyte by introducing an additional pair of electrodes and an electrolyte reservoir exists. One of the electrodes (an anode) is located in the electrolyte reservoir, whereas the other electrode (a cathode) is located in the sensing chamber. By applying 1.5V dc potential across the two additional electrodes, active hydrogen generated on the cathode in the sensing chamber reacts with dissolved oxygen to form water. This technique requires a complicated sensor structure, and in some cases, gives rise to a negative sensor output signal due to the reaction of active hydrogen on the sensing electrode.